1. Field
This invention is generally related to telemetry systems, and in particular, to telemetry systems utilized by high-speed platforms.
2. Related Art
Known telemetry systems for high-speed platforms (“HSPs”) such as, for example, launch vehicles, upper stages of launch vehicles, high-speed missiles, hypersonic vehicles, and other similar types of vehicles have known issues in dealing with the Doppler effects created by the HSPs. Specifically, a HSP introduces Doppler and Doppler rate issues in a transmitted telemetry stream that the HSP transmits to a remote station such as, for example a communication satellite (such as, for example, a relay satellite). These Doppler and Doppler rate issues generally cause complications in a receiver at the remote station because the frequency bandwidth of the receiver and the associated tracking loop, or loops, are generally not capable of dealing with the result of a high Doppler situation affecting the apparent received frequency of the telemetry stream. As such, these Doppler and Doppler rate issues generally cause the receiver to lose receiver lock on the telemetry stream coming from the HSP (generally known as a telemetry dropout) when the Doppler rate change is too high.
In FIG. 1, a system block diagram is shown of an example of an implementation of a known telemetry system 100 for a HSP 102 where the HSP 102 is an upper-stage of a launch vehicle. In this example, the telemetry system 100 includes a relay satellite 104, as the remote station above the surface of the Earth 106, and a ground station 108. The HSP 102 is shown as having been launched from a ground-based launch site 110 via a launch path 112 and then traveling along a flight path 114 (i.e., a travel path after separating from the lower-stage 116 of the launch vehicle) above the surface of the Earth 106 but below the altitude 118 of the relay satellite 104. In this example, the relay satellite 104 may be a near geostationary geosynchronous communication satellite orbiting at an altitude 118 of approximately 35,800 kilometers and the HSP 102 may be traveling along the flight path 114 at a velocity 120 that is, for example, greater than Mach 1. As an example, the relay satellite 104 may be a Tracking and Data Relay Satellite (“TDRS”) spacecraft operated by the United States (“U.S”) National Aeronautics and Space Administration (“NASA”) operating at in the S, Ku, and Ka frequency bands with a corresponding ground station 108 located at either White Sands Complex (“WSC”) in southern New Mexico, Guam Remote Ground Terminal (“GRGT”), or Network Control Center located at Goddard Space Flight Center in Greenbelt, Md. The HSP 102 includes a platform transmitter 122 that is in signal communication with the relay satellite 104 via a signal path 124 and the relay satellite is also in signal communication with the ground station 108 via signal path 126, where the signal path 124 is a satellite uplink and the signal path 126 is a satellite downlink.
In an example of operation, the HSP 102 is moving along the flight path 114 at a high velocity 120 towards the relay satellite 104. While approaching the relay satellite 104, the HSP 102 is transmitting a telemetry stream 128 of telemetry data related to measurements made and other data collected at the HSP 102 by the on-board sensors of the HSP 102. This telemetry stream 128 is generally received by the relay satellite 104 via the satellite uplink 124 and then converted into a downlink telemetry stream 130 that is transmitted to the ground station 108 via the satellite downlink 126. It is appreciated by those of ordinary skill in the art that this conversion may include frequency and modulation conversations. Once received at the ground station 108, the telemetry data transmitted by the HSP 102 may be monitored and analyzed.
Unfortunately, as the HSP 102 approaches the perpendicular position 132 of the relay satellite 104 along the flight path 114, the Doppler rate affecting the transmitted telemetry stream 128 starts to change very rapidly to the point that it heavily taxes the ability of the tracking loop, or loops, in the receiver (not shown) to maintain a lock on the telemetry stream 128 and may generally result in a loss of lock and associated dropout of the telemetry stream 128. Based on how quickly the telemetry system 100 is designed to resynchronize utilizing satellite synchronization data such as, for example, satellite ephemeris data and transmitted synchronization world signals, a significant amount of valuable telemetry data may be irrevocably lost when a dropout of the telemetry steam 128 occurs.
This problem is increased when the HSP 102 reaches and passes the perpendicular position 132 of the relay satellite 104 along the flight path 114. This perpendicular position 132 is generally a Doppler transition position because prior to reaching the perpendicular position 132 of the replay satellite 104, the Doppler rate caused by the approaching HSP 102 is positive as long as the HSP 102 is still located at a position on the approaching side 134 of the perpendicular position 132 of the relay satellite 104 along the flight path 114. However, once the HSP 102 crosses the perpendicular position 132 to receding side 136 of the perpendicular position 132 of the relay satellite 104 along the flight path 114, the Doppler rate changes approximately instantaneous to a negative Doppler rate because the HSP 102 is receding away from the relay satellite 104. This translation position 132 is known as a Doppler blind spot and will generally result in loss of lock of the telemetry stream 128.
Known approaches to solve this problem are complex and generally involve complex designs of multiple tracking loops that increase the tracking loop bandwidths but reduce the receiver sensitivity and/or require detailed a priori knowledge of the Doppler and Doppler rate as a function of the specific trajectory (i.e., the flight path 114) of the HSP 102. As a result, this known approach requires prior coordination and planning with the communication satellite system organization (“CSSO”) controlling and operating the relay satellite 104. As an example, currently the TDRS system is capable of creating a model for a flight path 114 of the HSP 102 that includes a planned Doppler schedule of the flight path 114 and compensates for the Doppler and Doppler rate issues at the relay satellite 104 and ground system 108. Unfortunately, this approach is controlled and operated on the CSSO (i.e., NASA in this example) side (i.e., the remote station side) and must be designed and implemented by CSSO. Additionally, if something happens and the HSP 102 does not fly at the appointed time that the model is designed for, the model is no longer valid and a new model must be designed for a future flight. In certain situations, this delay may be unacceptable.
As such, there is a need for a new system and method capable of compensating for high Doppler in a telemetry system where the high Doppler is caused by an HSP and does not have the limitations associated with the current systems.